Move user administration to a dropdown button to match the behavior of member's listing.

Issue

See originating issue: https://gitlab.com/gitlab-org/gitlab/-/issues/327378

The Members Admin screen is unnecessarily messy. The dialog containers are redundant with the modal popup warnings that follow clicking an administrative button.

Proposal

Let's move the user administration buttons to the primary action button row to mimic the same UX of the members list.

Current screens:

Memeber's list	Current User detail screen

Updates

Confirm user	zero state	drop down	modal